PC bond cleavage of (silox)3NbPMe3 (silox = tBu3SiO) under dihydrogen leads to (silox) 3Nb=CH2, (silox)3Nb=PH or (silox) 3NbP(H)Nb(silox)3, and CH4

Article abstract of DOI:10.1021/ja057747f

Photolysis of the equilibrium mixture (silox)₃NbPMe₃ (1) + H₂ (1-3 atm) ? (silox)₃Nb(H_eq)₂ (2e, tbp)/(silox)₃Nb(H_t)₂ (2t, pseudo-T_d) + PMe₃ causes PC bond cleavage. Depending on conditions, various amounts of (silox)₃Nb=CH₂ (3), (silox)₃Nb=PH (5-H), (silox)₃Nb=PMe (5-Me), (silox)₃Nb=P(H)Nb(silox)₃ (9, precipitated if N₂ is present; X-ray), (silox)₃NbH (4, active only through equilibration with 2e,t), and CH₄ are produced. Addition of PH₃ to 1 provides an independent route to 5-H; its deprotonation gives [(silox)₃NbP]Li (6), whose methylation yields 5-Me. Early conversion 3:5-H ratios of ～3:1 suggest that initial PC bond activation is slow relative to subsequent PC bond cleavages. Addition of HPMe₂ and H₂PMe to 1 generates (silox)₃HNbPMe₂ (7) and (silox)₃HNbPHMe (8), respectively, and both degrade faster than PMe₃. A mechanism based around sequential PC or CH oxidative addition, followed by 1,2-elimination events, is proposed. The limiting step in the decomposition of all PMe₃ is a slow hydrogenation of 3 to regenerate 2e,t and produces CH₄. Hydrides 2e,t are likely to be the photolytically active species. Copyright

Full text of DOI:10.1021/ja057747f

Published on Web 02/02/2006
t
PC Bond Cleavage of (silox)3NbPMe3 (silox ) Bu3SiO) under Dihydrogen
Leads to (silox)3NbdCH2, (silox)3NbdPH or (silox)3NbP(H)Nb(silox)3, and CH4
Kurt F. Hirsekorn, Adam S. Veige, and Peter T. Wolczanski*
Department of Chemistry & Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853
Received November 14, 2005; E-mail: ptw2@cornell.edu
The formation of carbon-heteroatom bonds (e.g., PC bonds)1-4
Scheme 1
has been a focal point of organometallic reactivity studies for the
past decade. Sometimes, it is also important to know how C-X
bonds can be broken. Phosphorus-carbon bond cleavage, which
is potentially useful with regard to the destruction of toxic agents,
is often deleterious to transformations catalyzed by ((R/R′)3P)nMLmXo.
Substantial precedent exists for PR cleavage when R ) aryl,5-9
yet only a limited number of phosphorus-alkyl bond scissions are
known,10-15 and the majority involve chelating ligands (e.g., Ph2-
PCH2Ph2).14,15 While investigating the stability of (silox)3NbPMe3
(1)16 under dihydrogen, we discovered that the PC bonds of PMe3
were thermally severed, and that ambient light assisted the
degradation. Herein we report our initial investigation of the thermal
and light-dependent hydrogenation of PMe bonds.
When exposed to H2 in hydrocarbon solvents, pseudo-Td, indigo
(silox)3NbPMe3 (1, S ) 1) equilibrated (K296 ∼ 1.4; ∆G° ) -0.2
kcal/mol) with two dihydrides, tbp (silox)3Nb(Heq)2 (2e) and pseudo-
Td (silox)3Nb(Ht)2 (2t), which were present in a 1.2:1.0 ratio.
Scheme 1 reveals structures that are based on calculations of
(HO)3NbH2 models (2e′, d(NbH) ) 1.78 Å, d(H‚‚‚H) ) 3.11 Å;
2t′, d(NbH) ) 1.77 Å, d(H‚‚‚H) ) 1.92 Å), which are consistent
with the rapid dissociative exchange of 2t with H2 (kf ∼ 1.0(5)
s-1, ∆Gq296 ) 17.4(2) kcal/mol), and the greater width of its hydride
resonance (2t, δ 14.08, ν1/2 ∼ 160 Hz; 2e, δ 11.39, ν1/2 ∼ 80 Hz)
3:5-H ratio suggests that, upon initial activation of PMe3, subsequent
PC bond cleavages are swift (i.e., 4 1 f 3 3 + 5-H + H2).
Scale-up of the PC bond cleavage caused subtle changes in the
product distribution. In a run monitored after 2 weeks, ∼10%
(silox)3NbdPH (5-H) was accompanied by ∼10% (silox)3Nbd
PMe (5-Me, 31P NMR δ 553 (br)), a green component alternatively
prepared from [(silox)3NbP]Li (6) and MeI,20 in addition to ∼40%
(silox)3NbdCH2 (3), and (silox)3NbH (4, ∼40%). In other reactions
of 3 and 4 weeks, the 4:3:5-H:5-Me ratio varied from 3:3.5:0:1.0
to 1.0:9.0:<0.5:0, respectively. Initial PC bond cleavage appears
to depend on the reaction vessel (wall thickness), and thus the
available UV light, but seems to be relatively insensitive to [Nb]tot
and pH2 (1-3 atm). In all instances, 5-H grew in at the expense of
5-Me, and CH4 was producedsas much as 1.9 equiv after 2 weeks,
and 2.7 equiv after 6 months according to Toepler pump measure-
ments.
The ultimate loss of all three methyl groups from PMe3 suggested
a stepwise removal. Using the method of Abbayes et al.,24 HPMe2
and H2PMe were prepared in situ and added to (silox)3NbPMe3
(1). With the former, a swift reaction afforded (silox)3HNbPMe2
(7), but 7 subsequently degraded (t1/2 ∼ 12 h) to produce an initial
∼1:1 ratio of 3:5-Me and CH4. With H2PMe, (silox)3HNbPHMe
(8, t1/2 ∼ 30 min) was generated, but precedented H2 loss9,19
predominated to give 5-Me in 60% yield, whereas PC activation
occurred to a lesser extent: 20% 5-H, 20% 3, and CH4. The
alkylphosphide hydrides25,26 are likely resting states preceding PC
or CH bond activation, and both demethylations occur on a time
scale faster than that of the initial PC/CH bond activation of PMe3.
It is tempting to conclude that the 7 f 5-Me transformation occurs
via PC bond activation to give (silox)3MeNbPHMe (7′) followed
by 1,2-MeH elimination, but one cannot rule out rearrangement of
7 via (silox)3HNbCH2PHMe f (silox)3(H2MeP)NbdCH2 f 7′.
A compilation of these observations is presented in Scheme 1.
Light-facilitated oxidative addition of either a PC or CH bond27
affords transients that are subject to 1,2-HPMe2 elimination and
formation of (silox)3NbdCH2 (3). The degradation of HPMe2 occurs
via substitution of PMe3 from 1 (or H2 from 2e,t) to give
1
in the H NMR spectrum; quadrupolar broadening is expected to
exert greater influence in the pseudo-cylindrically symmetric 2t.17
Thermal (60 °C) degradation of 1 in the darkswith or without
H2 presentsafforded ∼20% magenta (silox)3NbdCH2 (3, 1H (13C-
{1H}) NMR δCH 9.22 (216.35), JCH ) 135 Hz), which was
2
alternately prepared via addition of Ph3PdCH2 to 1.18 In NMR tubes
in ambient light (23 °C), PC bond cleavage occurred slowly without
H2, and ∼50% 3 formed over the course of 4 months along with a
paramagnetic product tentatively identified as purple (silox)3NbH
(4, ν(NbH/D) ) 1730/1248 cm-1), which was generated indepen-
dently upon hydrogenation of (silox)3Nb(olefin), ∼10% (silox)3Nbd
PH (5-H, 31P NMR δ 515.26, JPH ) 75 Hz), and CH4. The addition
of PH3 to 1 cleanly provided 5-H,9,19 which was deprotonated with
LiN(SiMe3)2 to generate the yellow phosphide20-22 [(silox)3NbP]-
Li (6, 93Nb NMR δ 463, d, JNbP ∼ 550 Hz; 31P NMR δ 790, m,
ν1/2 ∼ 5800 Hz)23 in useful quantities.
In NMR tubes in ambient light, solutions of 1 + H2 a 2e,t +
PMe3, which were stable in the dark at 23 °C, produced a ∼4:3:1
ratio of (silox)3NbH (4):(silox)3NbdCH2 (3):(silox)3NbdPH (5-
H) over 10 days along with methane. We conclude that 2e,t has a
greater light sensitivity toward PC bond cleavage than does 1, and
that its singlet excited states may be critical to facile PC activation.
With a 450 W Hanovia (Hg) light source, PC bond cleavage
diminished to ∼15%, and ∼80% 4 was generated after 3 days.
Monohydride 4 appears to be thermodynamically favored, but
independent reactions showed that it does not effect PC bond
cleavage unless additional H2 is present (4 + 1/2H2 a 2e,t). The
9
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J. AM. CHEM. SOC. 2006, 128, 2192-2193
10.1021/ja057747f CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
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2
2
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1
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Acknowledgment. We thank the NSF (CHE-0415506), Thomas
R. Cundari, Rachel Combs (calculations), and Emil B. Lobkovsky
(X-ray).
Supporting Information Available: Spectral and analytical data,
CIF file for 9, and experimental and computational procedures. This
material is available free of charge via the Internet at http://pubs.acs.org.
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